Heat exchanger tube

Abstract

The invention relates to a heat exchanger tube consisting of copper or a copper alloy, having a tube wall and an outer side and an inner side, wherein at least 90% by volume of columnar or ellipsoidal crystallites are embedded in the microstructure, and the tube wall has a hardness HV0.2 of at least 120 over the entire cross-section.

Description

[0001] Wieland-Werke AG 89070 Ulm, March 11, 2026 Bt / Eb GP3470

[0002] Description

[0003] heat exchanger tube

[0004] The invention relates to a heat exchanger tube according to the preamble of claim 1.

[0005] Heat transfer occurs in many areas of refrigeration and air conditioning technology, as well as in process and energy engineering. In these fields, heat exchangers with tubes are frequently used for heat transfer. In many applications, a liquid or gaseous medium flows on the inside of the tube, which is cooled or heated depending on the direction of the heat flow. The heat is then transferred to or extracted from the medium on the outside of the tube.

[0006] To enable heat transfer between the heat-emitting and heat-absorbing mediums, the temperature of the heat-emitting medium must be higher than the temperature of the heat-absorbing medium. This temperature difference is called the driving temperature difference. The higher the driving temperature difference, the more heat can be transferred. Conversely, it is often desirable to keep the temperature difference low, as this is advantageous for process efficiency.

[0007] It is known that structuring the heat transfer surface can intensify heat transfer. This allows more heat to be transferred per unit area compared to a smooth surface. Furthermore, it is possible to reduce the driving temperature difference and thus make the process more efficient. In metallic heat exchanger tubes, structuring the heat transfer surface is often achieved by forming fins or similar elements from the tube wall material. These integrally formed fins have a strong metallic bond with the tube wall and can therefore transfer heat optimally.

[0008] A commonly used type of heat exchanger is the shell and tube heat exchanger. These devices often employ tubes that are structured on both their inner and outer surfaces.

[0009] Structured heat exchanger tubes for shell and tube heat exchangers typically have at least one structured section, as well as smooth end pieces and possibly smooth intermediate pieces. The smooth end pieces and intermediate pieces define the structured sections.

[0010] For special applications, such as gas chillers, the operating range of the refrigerants used extends to extremely high pressures of over 100 bar, far exceeding the typical pressure of up to 35 bar for CFC and HFC safety refrigerants. However, depending on the application, a permissible pressure of around 50 bar is also required for evaporators.

[0011] These pressure requirements are difficult to meet with copper tubes made of Cu-DHP, which are typically used in heat exchangers operated with CFC and HFC safety refrigerants, because Cu-DHP requires very thick tube walls, with correspondingly negative effects on workability, the weight of the heat exchanger, and the equipment costs. Instead, it is now state of the art to use tubes made of steel, copper-nickel, or stainless steel, with which the aforementioned pressures can be managed relatively easily.

[0012] However, the steel, copper-nickel and stainless steel pipes used so far also have significant disadvantages compared to copper in terms of processability, efficiency and cost.

[0013] Advantageous solutions for a condenser or evaporator tube, consisting of a copper-iron-phosphorus alloy which may also contain zinc and tin for strength enhancement, are already described in the applicant's publication DE 102006013384 B4. Such heat exchanger tubes can be used, for example, as gas cooler, condenser, or evaporator tubes in a CO2-operated refrigeration machine or heat pump. The working medium, CO2, flows on the inside of the heat exchanger tubes and, depending on the temperature conditions of the specific application, exhibits a pressure level that differs significantly from the pressures known for CFC and HFC safety refrigerants and places high demands on the pressure resistance of the tubes used.

[0014] Consequently, it would be desirable to achieve the high requirements for the pressure resistance of the pipes used, even in the context of the often-used pure copper pipes.

[0015] The invention is based on the objective of further developing a high-performance heat exchanger tube for condensation or evaporation with regard to improving pressure stability.

[0016] The invention is described by the features of claim 1. The further referenced claims relate to advantageous embodiments and further developments of the invention.

[0017] The invention includes a heat exchanger tube made of copper or a copper alloy, with a tube wall and an outer and an inner surface, wherein at least 90 vol.% of columnar or ellipsoidal crystals are embedded in the microstructure, and the tube wall has a hardness HV0.2 of at least 120 over the entire cross-section.

[0018] Preferably, the copper content in the material of the heat exchanger tube is at least 99.90 wt.%.

[0019] According to the invention, the pipe's outer diameter can range from 3 to 50.8 mm. An advantageous ratio of wall thickness to pipe outer diameter can be selected, for example, in the range of 0.021 to 0.08. This results in pipe wall thicknesses that are in a similar range to those of the copper pipes made of Cu-DHP commonly used today for HFC safety refrigerants.

[0020] The particular advantage lies in the fact that the structural elements according to the invention allow for thin walls even under high pressure loads, thus enabling significant material savings and increasing heat transfer efficiency, resulting in weight and cost advantages. Due to the increased heat transfer efficiency compared to the prior art, the required refrigerant charge can be reduced, which, with the predominantly used chlorine-free safety refrigerants today, can represent a considerable portion of the total system costs. With toxic or flammable refrigerants, which are generally only used in special cases, reducing the charge can also decrease the potential hazard.

[0021] In a preferred embodiment of the invention, at least 75% of the crystallites can be aligned in their longitudinal extent in the direction of the pipe axis.

[0022] In particular, it is especially advantageous if at least 90% of the crystallites are oriented along the pipe axis, particularly in areas of the pipe wall subjected to tensile stress under high pressure. Advantageously, the crystallites can have a length-to-width ratio of at least 5:1. This longitudinal orientation results in a microstructure that is particularly resistant to high pressure.

[0023] It is also advantageous that globular crystallites are present in the microstructure at a maximum of 5 vol.%. A certain proportion of globular crystallites results in a favorable microstructure that still allows for easy forming and, in particular, further processing of the outer or inner surface.

[0024] In an advantageous embodiment of the invention, the pipe material, i.e., the material of the heat exchanger pipe, can have a yield strength R pThe yield strength must exceed 200 MPa, preferably 400 MPa. This yield strength allows for a reduction in pipe wall thickness compared to known prior art solutions, resulting in significant material savings and improved processability. For example, a pipe with an outside diameter of 9.52 mm and an operating pressure exceeding 100 bar requires a maximum pipe wall thickness of 0.55 mm, representing a material saving of over 40% compared to conventional Cu-DHP pipes.

[0025] In an advantageous embodiment of the invention, the pipe material can have a tensile strength R mThe pipe material must have a yield strength exceeding 300 MPa, preferably exceeding 400 MPa. The resulting resistance of the pipe material is designed for a pressure level significantly higher than that of CFC and HFC safety refrigerants. This also results, for example, in a pipe with an outer diameter of 7.94 mm (5 / 16 inch) and an operating pressure of at least 100 bar, requiring a maximum pipe wall thickness of 0.28 mm, thus achieving a material saving of more than 45% compared to known solutions. Advantageously, the pipe material can have a yield strength Rto,s exceeding 200 MPa, preferably exceeding 400 MPa. For applications where pipe collapse resistance is critical, wall thicknesses of up to 0.65 mm can be advantageous for 19.05 mm (5 / 16 inch) refrigeration pipes at the specified yield strengths.

[0026] In a preferred embodiment of the invention, the outer surface and / or the inner surface of the heat exchanger tube can be structured. In conjunction with the inventive solution, the heat transfer coefficient and thus the heat transfer performance can be increased if the outer surface of the tube is structured. Further advantages arise if the inner surface of the tube is also structured. In particular, with the tubes according to the invention, the challenges regarding the external and / or internal forming of hard pre-material can be advantageously solved.

[0027] In a particularly preferred embodiment, the pipe wall of the pre-material in a structured heat exchanger tube can have a hardness HV0.2 of at least 130 over the entire cross-section before structuring.

[0028] On the one hand, hardness is a crucial parameter for the compressive strength of a pipe. On the other hand, the formability of the pipe itself, and especially in conjunction with surface texturing to improve thermal conductivity within the specified hardness values, must be considered. The condition of the pipe wall of the pre-material can be determined on the smooth end pieces and any smooth intermediate sections of the structured heat exchanger pipe.

[0029] In a further advantageous embodiment of the invention, the pipe material can exhibit a thermal conductivity exceeding 340 W / mK. Since the microstructure consists of at least 90 vol% columnar crystallites, a certain anisotropy in the thermal conductivity of the pipe material arises along the longitudinal axis of the pipe and perpendicular to it. The respective number of grain boundaries in the different spatial directions makes a significant contribution to this anisotropy. Thus, the thermal conductivity can be optimized to the desired level by adjusting the orientation of the columnar crystallites.

[0030] Exemplary embodiments of the invention are explained in more detail with reference to the schematic drawing.

[0031] It shows:

[0032] Fig. 1 shows a schematic partial view of a longitudinal section of a heat exchanger tube, cut along the tube axis with an outer and inner structure.

[0033] Fig. 1 shows a partial view of a longitudinal section of a heat exchanger tube 1 parallel to the tube axis X with a structure to increase thermal conductivity on the outside A and the inside I.

[0034] An outer structure 110 has been machined from the outer pipe wall 11 to increase heat transfer. On the inner side I, continuous, helically circumferential inner ribs have also been formed from the inner pipe wall 13 as an inner structure 111. The pipe axis X runs in a horizontal direction. The microstructure of the heat exchanger tube 1 consists almost exclusively of columnar crystallites 2 with a length-to-width ratio of over 5:1.

[0035] In the region of the central pipe wall 12, columnar crystallites 2 are oriented particularly parallel to the pipe axis X. It is precisely these crystallites 2, oriented parallel to the pipe axis X, that result in the advantageous yield strength, tensile strength, and tensile strength. Reference numeral list

[0036] 1 heat exchanger tube

[0037] 11 outer pipe wall

[0038] 12 middle pipe wall

[0039] 13 inner pipe wall

[0040] 110 Exterior structure

[0041] 111 Internal structure

[0042] 2 Crystallites

[0043] A Outside

[0044] Inside

[0045] X pipe axis

Claims

Patent claims 1. Heat exchanger tube (1) consisting of copper or a copper alloy, with a tube wall and an outer surface (A) and an inner surface (I), characterized by - that at least 90 vol% of the structure contains columnar or ellipsoidal crystallites (2), and - that the pipe wall has a hardness HV0.2 of at least 120 across its entire cross-section.

2. Heat exchanger tube (1 ) according to claim 1 , characterized in that at least 75% of the crystallites (2) are aligned in their longitudinal extent in the direction of the tube axis (X).

3. Heat exchanger tube (1) according to claim 1 or 2, characterized in that the crystallites (2) have a length-to-width ratio of at least 5:

1.

4. Heat exchanger tube (1) according to one of claims 1 to 3, characterized in that crystallites (2) with globular shape are present in the microstructure at most up to 5 vol.%.

5. Heat exchanger tube (1) according to one of claims 1 to 4, characterized in that the tube material has a yield strength R p 0.2 has a tensile strength exceeding 200 MPa, preferably exceeding 400 MPa.

6. Heat exchanger tube (1) according to one of claims 1 to 5, characterized in that the tube material has a tensile strength R m has a pressure of over 300 MPa, preferably over 400 MPa.

7. Heat exchanger tube (1) according to one of claims 1 to 6, characterized in that the tube material has a yield strength Rto.s above 200 MPa, preferably above 400 MPa.

8. Heat exchanger tube (1) according to one of claims 1 to 7, characterized in that the outside (A) and / or the inside (I) of the heat exchanger tube (1) is structured.

9. Heat exchanger tube (1) according to claim 8, characterized in that, prior to structuring, the tube wall of the pre-material has a hardness HV0.2 of at least 130 over the entire cross-section.

10. Heat exchanger tube (1) according to one of claims 1 to 9, characterized in that the tube material has a thermal conductivity above 340 W / mK.

Images